U.S. patent application number 14/596288 was filed with the patent office on 2016-07-14 for dual role antenna assembly.
The applicant listed for this patent is SKYWAVE MOBILE COMMUNICATIONS INC.. Invention is credited to Phil Lafleur, David Roscoe.
Application Number | 20160204519 14/596288 |
Document ID | / |
Family ID | 55072589 |
Filed Date | 2016-07-14 |
United States Patent
Application |
20160204519 |
Kind Code |
A1 |
Lafleur; Phil ; et
al. |
July 14, 2016 |
DUAL ROLE ANTENNA ASSEMBLY
Abstract
A dual role antenna assembly operable for use with GEO and
LEO/MEO satellites has at least two curled inverted-F substantially
omnidirectional antennas mounted on a ground plane. The antennas
have asymmetrical gain patterns favoring certain sectors and are
oriented such that the favored sectors of the different antenna
face different directions. A controller selects the antenna for
connection to an RF front-end in accordance with predetermined
performance criteria.
Inventors: |
Lafleur; Phil; (Ottawa,
CA) ; Roscoe; David; (Ottawa, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SKYWAVE MOBILE COMMUNICATIONS INC. |
Ottawa |
|
CA |
|
|
Family ID: |
55072589 |
Appl. No.: |
14/596288 |
Filed: |
January 14, 2015 |
Current U.S.
Class: |
342/357.39 ;
343/893; 455/13.3 |
Current CPC
Class: |
H01Q 1/38 20130101; H01Q
9/0428 20130101; H01Q 9/0421 20130101; H01Q 9/0442 20130101; H01Q
5/50 20150115; H01Q 3/24 20130101 |
International
Class: |
H01Q 21/28 20060101
H01Q021/28; G01S 19/01 20060101 G01S019/01; H01Q 5/30 20060101
H01Q005/30; H01Q 9/04 20060101 H01Q009/04; H01Q 1/38 20060101
H01Q001/38 |
Claims
1. A dual role antenna assembly operable for use with GEO and
LEO/MEO satellites, comprising: a ground plane; at least two curled
inverted-F substantially omnidirectional antennas mounted on the
ground plane, said antennas having asymmetrical gain patterns
favoring certain sectors, and said antennas being oriented such
that the favored sectors of the different antenna face different
directions, and an RF beam selection switch for selectively
connecting said antenna to an RF front-end; and a controller
controlling said RF beam selection switch to in accordance with
predetermined performance criteria.
2. A dual role antenna assembly as claimed in claim 1, wherein the
gain patterns of said antenna are tilted in relation to the horizon
with said antennas having optimum low elevation performance facing
in different directions.
3. A dual role antenna assembly as claimed in claim 2, comprising
at least two said antennas, and wherein said different directions
for a pair of said antennas are diametrically opposed.
4. A dual role antenna assembly as claimed in claim 1, wherein the
controller is programmed to give priority to GEO satellites.
5. A dual role antenna assembly as claimed in claim 1, wherein the
controller is programmed to share the selected antenna with both
GEO and LEO/MEO satellites in a half-duplex manner.
6. A dual role antenna assembly as claimed in claim 1, wherein the
controller is programmed to use the selected antenna for both GEO
satellites and the or one of the other antennas for LEO/MEO
satellites.
7. A dual role antenna assembly as claimed in claim 1, further
comprising a received signal strength monitor for providing a
received signal strength indication, and wherein the predetermined
performance criteria comprise the received signal strength
indication.
8. A dual role antenna assembly as claimed in claim 7, wherein the
received signal strength indication is based on signals received
from a GEO satellite.
9. A dual role antenna assembly as claimed in claim 1, wherein said
antennas are tunable between frequency sub-bands, and further
comprising a frequency switch associated with each said antenna and
operative to switch between the sub-bands.
10. A dual role antenna assembly as claimed in claim 9, wherein
each said frequency switch is controlled by said controller.
11. A dual role antenna assembly as claimed in claim 9, wherein
said antennas have multiple feed points corresponding to the
different sub-bands.
12. A dual role antenna assembly as claimed in claim 9, wherein the
ground plane lies on a printed circuit board, and each said
frequency switch is mounted on the printed circuit board in close
proximity to the antennas.
13. A dual role antenna assembly as claimed in claim 9, wherein
said frequency switch associated with each said antenna is mounted
inside a dielectric form forming part of said antenna.
14. A dual role antenna assembly as claimed in claim 1, wherein the
height of the antenna is at least 12 mm.
15. A dual role antenna assembly as claimed in claim 1, wherein the
curled inverted-F antenna is mounted on an elliptical dielectric
form.
16. A method of controlling dual role antenna assembly operable for
use with GEO and LEO/MEO satellites, comprising at least two curled
inverted-F substantially omnidirectional antennas mounted on the
ground plane, said antennas having asymmetrical gain patterns
favoring certain sectors, and said antennas being oriented such
that the favored sectors of the different antenna face different
directions, said method comprising: measuring a performance
indication for each antenna; and selecting as a primary antenna the
antenna with the best performance indication.
17. A method as claimed in claim 16, wherein the primary antenna is
shared with the GEO and LEO/MEO satellites in a half duplex
manner.
18. A method as claimed in claim 16, wherein the other antenna is
used for the LEO/MEO satellites.
19. A method as claimed in claim 16, wherein the performance
indication is the received signal strength indication.
20. A method as claimed in claim 16, wherein the performance
indication is the received signal strength indication of a GEO
satellite.
21. A method as claimed in claim 16, wherein the antenna are
tunable stepped across a frequency band.
22. A method as claimed in claim 21, wherein the antennas have
multiple feed points, and different feed points are selected for
different frequency sub-bands.
23. An antenna comprising: a dielectric form of elliptical cross
section; and conductive strips peripherally mounted on said
dielectric form to provide a curled inverted-F substantially
omnidirectional antenna, said antenna having an asymmetrical gain
pattern favoring certain sectors.
24. An antenna as claimed in claim 23, wherein said dielectric form
has major and minor axis radii of 11 mm and 7 mm, respectively, and
a height of 12 mm.
25. An antenna as claimed in claim 23, wherein said dielectric form
is hollow.
Description
FIELD OF THE INVENTION
[0001] This invention relates to the field of antenna, and more
particularly to a dual role antenna assembly operable for use with
use with geostationary earth orbit (GEO) and low earth orbit/medium
earth orbit (LEO/MEO) satellite constellations, and to a method of
controlling such an antenna.
BACKGROUND OF THE INVENTION
[0002] Designers of mobile satellite communication antenna systems
are faced with a number of conflicting system requirements. The
link budget benefits from higher gain, but an omnidirectional
pattern is best from a system coverage perspective. The antennas
should be low profile and yet have good low elevation angle
performance. They should also be small and yet have sufficiently
wide bandwidth.
[0003] Exploring these trade-offs typically leads to the selection
of patch antenna technology if maintaining a low profile is
critical, or helical antennas if profile is less important but low
elevation angle performance is vital. Furthermore, maintaining low
cost is critical for commercial applications.
[0004] While a patch antenna is typically low profile, there are a
number of problems with the patch antenna, namely the low elevation
angle performance is not good, in the case where the antenna and
transceiver are integrated onto a single PCB, it takes up a large
amount of space on the top side of the transceiver, forcing the
electronics to the bottom side, limiting miniaturization. Moreover,
the patch antenna requires a substantial ground plane further
miniaturization and there is a difficult bandwidth/volume
trade-off.
[0005] While a helical antenna typically has good low elevation
angle performance, there are a number of problems with the helical
antennas. They have a relatively high profile, typically a
significant fraction of a wavelength in height, the radiation
pattern is typically impaired by the ground plane/electronics PCB,
and they take up a large amount of space on the top side of the
transceiver
[0006] Another substantially omnidirectional antenna is the curled
inverted-F antenna (CIFA). This is essentially an inverted-F
antenna with a curled-end. With the curled end and optimized
placement and orientation in the corner of an optimally sized
ground plane, reasonably good circular polarization performance can
be achieved. One example of such an antenna is sold by TE
Connectivity under part no. 1513634-1. This GPS antenna is about 6
mm in height and 16 mm in diameter.
[0007] While this antenna is compact and lends itself well to
integration along with other components on the same PCB, it has a
number of limitations, including narrow bandwidth (only about 22
MHz for the 1513634-1), and intrinsic radiation pattern issues,
such as a tilted beam with non-uniform RHCP (Right Hand Circular
Polarization) coverage, which would mitigate against using this
kind of antenna for some GEO applications.
[0008] Diversity antenna systems are known, for example, as
described in U.S. Pat. No. 8,305,270 to mitigate multipath fading,
particularly deep fades. Known diversity systems do not improve
system performance in situations where fading is not a factor.
SUMMARY OF THE INVENTION
[0009] Embodiments of the invention employ a diversity antenna
system that uses a tilted radiation pattern to enhance low
elevation angle gain for one higher priority satellite, while
maintaining sufficient omnidirectionality to function well with the
remaining satellites.
[0010] According to the present invention there is provided a dual
role antenna assembly operable for use with GEO and LEO/MEO
satellites, comprising a ground plane; at least two curled
inverted-F substantially omnidirectional antennas mounted on the
ground plane, said antennas having asymmetrical gain patterns
favoring certain sectors, and said antennas being oriented such
that the favored sectors of the different antenna face different
directions, and an RF beam selection switch for selectively
connecting said antenna to an RF front-end; and a controller
controlling said RF beam selection switch to in accordance with
predetermined performance criteria.
[0011] It will be understood that substantially omnidirectional in
this context means that the antenna generally has all round
coverage to receive (or transmit) signals from any direction
outside of a small exclusion zone where reception (or transmission)
is impaired. However, a radiation pattern is never completely
uniform and in practice one direction has higher gain. Also, the
gain pattern is generally tilted relative to the horizon, so that
one sector will have better low elevation performance.
[0012] In one embodiment, for example for a dual GNSS/Satellite
Communication (SATCOM) environment, the controller selects the
antenna with the best RSSI (Received Signal Strength Indication)
for the geostationary satellite communications system (GEO). A
number of other system parameters could be used to control the
switching. The performance could also be measured against some
predetermined value.
[0013] The GNSS system then shares the selected antenna in a half
duplex fashion. Because of frequency band proximity in the
preferred embodiment, the same receive chain front-end is shared
between GNSS and GEO. An alternative approach is to use the other
antenna or one of the other antennas if there are more than two for
the GNSS system.
[0014] Further embodiments of the invention thus provide two or
more antenna elements in which GNSS and GEO front-ends, whether
shared or separate are connected to share the same element or use
different element according to predetermined selection
criteria.
[0015] The bandwidth limitations of the CIFA element can be partly
overcome by increasing the height the antenna, for example, by
doubling the height to 12 mm. Thus, the height of the curled
inverted-F antenna should be at least 12 mm for good bandwidth
performance in GEO systems with typical manufacturing tolerances.
However, in addition, multiple feed strips can be provided for the
antenna to optimize its performance for multiple sub-bands. An RF
switching module is provided in this case to switch between the
feed strips according to the required sub-band depending on the
particular frequency in use.
[0016] Further embodiments of the invention thus provide a
multiband antenna consisting of two or more feed strips which
enable switching to different frequency bands, creating a composite
bandwidth that is larger than the instantaneous bandwidth and a
multiple beam array (MBA) in which two or more substantially
omnidirectional antenna elements are switched in such a way as to
create a composite radiation pattern that has a more uniform
overall radiation pattern with less pronounced coverage gaps than a
single substantially omnidirectional element.
[0017] Unlike MBAs in the prior art, where the object is usually to
create a directional beam, in accordance with the present invention
the object of the MBA is to achieve omnidirectional coverage. The
composite radiation pattern is achieved by connecting the RF
front-end directly to the array element corresponding with the
desired beam pattern. The superposition of individual element
radiation patterns creates and an aggregate MBA radiation pattern.
Keeping only one element active at a time is necessary to ensure
that the MBA effective aperture area remains small, facilitating a
more omnidirectional radiation pattern.
[0018] In one embodiment, two multiple beam array antennas are
interchangeably used to communicate with two different satellites
or groups of satellites (constellations), one being higher priority
and the other being lower priority. For example, the higher
priority system could be a geostationary L-band two-way satellite
communication system with a single satellite and the lower priority
system could be a medium earth orbit L-band constellation such as
GPS, Galileo or GLONASS positioning systems.
[0019] To facilitate the design of the underlying antenna element,
it is preferable to have the systems involved operate in nearby
frequency bands. This enables simultaneous GEO/GNSS operation with
the same RF front-end.
[0020] The product configuration in the preferred embodiment is a
"GPS tracker" commonly used in a wide variety of telematics and
logistics applications.
[0021] In accordance with another aspect the invention provides a
method of controlling dual role antenna assembly operable for use
with GEO and LEO/MEO satellites, comprising at least two curled
inverted-F substantially omnidirectional antennas mounted on the
ground plane, said antennas having asymmetrical gain patterns
favoring certain sectors, and said antennas being oriented such
that the favored sectors of the different antenna face different
directions, said method comprising measuring a performance
indication for each antenna; and selecting as a primary antenna the
antenna with the best performance indication.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention will now be described in more detail, by way
of example only, with reference to the accompanying drawings, in
which:
[0023] FIG. 1 is a perspective view of an antenna element;
[0024] FIG. 2 is a perspective view of a two-antenna assembly
mounted on a printed circuit board;
[0025] FIG. 3 is a plan view of the two-antenna assembly showing
the switching components;
[0026] FIGS. 4a, 4b, and 4c are respectively sectional views
showing the radiation patterns for right hand and left hand
circular polarization for the single antenna shown in FIG. 1, where
FIG. 4a shows a first elevation cut, FIG. 4b shows a second
elevation cut, orthogonal to the cut of FIG. 4a and FIG. 4c shows
an azimuth cut;
[0027] FIG. 5 is a sectional view showing the radiation pattern for
the two-antenna assembly for right hand and left hand circular
polarization in the horizontal plane;
[0028] FIG. 6 is a perspective view of a four-antenna assembly
mounted on a printed circuit board;
[0029] FIG. 7 is a plan view of the four-antenna showing the
switching components;
[0030] FIG. 8 is a sectional view showing the radiation pattern for
the four-antenna assembly for right hand and left hand circular
polarization in the horizontal plane;
[0031] FIG. 9 shows an algorithm for determining the antenna
selection; and
[0032] FIG. 10 shows the frequency response for a tunable antenna
with two different feed points.
DETAILED DESCRIPTION OF THE INVENTION
[0033] The antenna element 2 shown in FIG. 1 is a curled inverted-F
antenna comprising an interrupted curled metal strip 4 mounted or
plated on the end of a hollow elliptical cylindrical dielectric
form 5 with a closed top 5a having arcuate slits 5b.
[0034] While an elliptical shape illustrated has been found to give
good performance, it will be understood that other shapes, such as
circular cylindrical, may be employed. The elliptical shape has the
added benefit of allowing a more space efficient use of on the top
side of a printed circuit board. An inverted F-antenna is
described, for example, in WO 2002029988, the contents of which are
herein incorporated by reference.
[0035] A small gap 6 is present between the ends of the interrupted
circular metal strip 4. One ground strip 7 and two metal feed
strips 8, 9, extend vertically from one end of the metal strip 4.
Ground strip 7 is connected to the ground plane provided by the
printed circuit board (PCB) 1. The other feed strips 8, 9
correspond to different frequency sub-bands.
[0036] A two-element antenna assembly shown in FIG. 2 comprises a
generally rectangular double sided printed circuit board 1,
providing a ground plane, on which are mounted two antenna elements
2a, 2b, each as shown in FIG. 1. The antenna elements 2a, 2b are
mounted at opposite corners of the printed circuit board 1, which
also has a grounded cover 10 housing components mounted on the
printed circuit board.
[0037] As shown in FIG. 3, the two feed strips 8, 9 of each antenna
element 2a, 2b are connected to an RF switch 11a, 11b located as
close as possible to the antenna element 2a, 2b, in this case
inside the dielectric form 5, by traces on the printed circuit
board 1. The RF switches 11a, 11b switch between different feed
strips 8, 9 for different frequency sub-bands.
[0038] The RF switches 11a, 11b are connected by traces on the
printed circuit board 1 to a beam-switching single-pole RF switch
13. The single-pole RF switch 13, which is connected to RF
front-end 14, is used to switch between different antenna elements
2a, 2b. The RF front-end 14 may be a transceiver for receiving GNSS
signals and transmitting and receiving communication signals. In
this example, it comprises a transmit module 16, receive module 17,
and RF switch 15 for switching between transmit and receive modules
16, 17. The receive module 17 also incorporates a signal strength
monitor 17a for obtaining a received signal strength indication
(RSSI).
[0039] The transmit module 16 is associated with the GEO satellites
since it is used to transmit signals via the satellites to a remote
ground station. The receive module 17 can be associated with either
the GNNS system or the GEO communications system as commanded by a
controller in the form of processor 19.
[0040] The RF switches 11a, 11b, 13, 15 and receive module 17 are
controlled by processor 19, which also receives a received signal
strength indication (RSSI) from RSSI monitor 17a in receive module
17.
[0041] As noted the GNSS positioning system, such as GPS, GLONASS,
or Galileo, uses the satellites in a low or medium earth orbit, and
which thus move relatively rapidly with respect to the receiver
unlike the GEO communications satellites, which are in
geostationary orbits.
[0042] The antenna elements 2a, 2b have an increased size relative
to known curled inverted-F antennas. In the exemplary embodiment
they are 12 mm in height and have major and minor axis radii of 11
mm and 7 mm, respectively. This gives them an increased bandwidth
of 130 MHz centered near the GPS frequency band. While scaling
volume increases bandwidth, an increase in height limits the
applicability of this approach in wider band systems where low
profile is required.
[0043] A single antenna 2 as shown in FIG. 1 mounted on a ground
plane (PCB 1) has a radiation pattern as shown in FIGS. 4a to 4c,
where FIG. 4a shows a first elevation cut, FIG. 4b shows a second
elevation cut, orthogonal to the cut of FIG. 4a , and FIG. 4c shows
an azimuth cut. The solid lines show the pattern for right hand
circular polarization (RHCP) while the dashed lines show the
pattern for left hand circular polarization (LHCP). In this
preferred embodiment, RHCP is the desired polarization.
[0044] These patterns show that the gain pattern is substantially
omnidirectional with slight bulge in one direction at low elevation
angles (FIG. 4a) forming a beam or favored direction. Low elevation
angle performance is the limiting factor in mobile satellite
communication systems, making the azimuth cut of the radiation
pattern (FIG. 4c) the focus of the present invention. The RHCP
radiation pattern is tilted as shown in FIG. 4a with a beam peak
typically at 165 degrees.
[0045] GEO system availability and reliability are more susceptible
to radiation pattern tilt than GNSS constellations. While generally
acceptable for GNSS constellations with multiple satellites in view
at different look angles, the degraded RHCP gain at low elevation
angles, such as zero degrees, does pose a problem for GEO systems
where the only available satellite might be unreachable due to the
low antenna gain.
[0046] Significantly, looking at the elevation cuts (FIGS. 4a, 4b),
it will be seen that the low elevation performance is also
directional. For example, looking at FIG. 4a, it will be seen that
the gain is near 2 dBic at 300.degree. but only -18 dBic at
120.degree., the corresponding position on the other side.
[0047] In the embodiment shown in FIG. 3 the two diametrically
opposed antenna array elements 2a, 2b are arranged at opposite
corners of the printed circuit board 1 with ground plane with the
favored directions for low elevation performance oriented in
diametrically opposed directions. In this embodiment, antenna 2a
has its favored direction for low elevation performance, i.e.
optimum low elevation gain as shown in FIGS. 4a, 4c facing to the
left and antenna element 2b has its favored direction oriented to
the right as shown by the solid arrows. In this way, the highest
gain sector of one element covers the lowest gain sector of the
other as shown in FIG. 5.
[0048] The antennas 2a, 2b thus have substantially isotropic
radiation patterns but whose radiation patterns are tilted to favor
low elevation angle radiation in one sector. As shown in FIG. 3,
these elements are arranged with 180 degree rotation relative to
each other. As a result, the radiation from antenna 2a is strongest
in the direction where antenna 2b is weakest and vice-versa. In
this way, when the beam selection algorithm, described in more
detail with reference to FIG. 9, run on processor 19 selects the
best antenna, even in situations where multipath fading is not an
issue, the system sees a net benefit to the link budget.
[0049] The reason that this is possible is that even though the
radiation patterns are tilted to provide improved low elevation
angle gain in one sector, the elements remain substantially
omnidirectional. They are carefully designed to be sufficiently
omnidirectional as to avoid significantly degraded system level
MEO/LEO/GNSS performance, as measured in this case by position
accuracy and 3-D fix availability. The composite antenna assembly
offers good aggregate radiation performance, especially at low
elevation angles. It should be noted however that having a tilted
beam is of no benefit to the positioning system because the
multiple satellites used in a given 3-D fix are distributed
throughout the solid angle above and around the antenna.
[0050] In alternative embodiment, there may be additional antenna
elements, for example, one antenna element 2a, 2b, 2c, 2d at each
corner as shown in FIGS. 6 and 7. These can be oriented to provide
optimum low elevation coverage. FIG. 8 shows a typically radiation
pattern for a 4-antenna system with the patterns rotated 90 degrees
for each antenna. It should be noted that adequate spacing between
MBA elements must be maintained to prevent radiation pattern
distortion at low elevation angles due to parasitic loading and
blockage effects. As a result the minimum viable PCB size for the
two-element configuration is smaller than the minimum viable
configuration for the four-element configuration. Two-element
configurations tend to be rectangular and four-element
configurations tend to be square like.
[0051] In the case of a two-element array, switch 15 is a TX/RX
SPDT switch, switch 13 is a beam selection SPDT switch, and
switches 11a, 11b are frequency band selection switches. In the
case of a four-element array, the SPDT beam selection switch 13 is
a SP4T beam selection switch. As noted all the RF switches are
controlled by the processor 19, and the beam selection switch
control depends on readings from the RSSI measurement module shown
here integrated in the receiver 17.
[0052] It is important that the frequency band selection switches
11a, 11b, 11c, 11d be located very close to the CIFA feed points.
In a dual-band configuration, the unused feed strip is loading the
antenna, acting like an open-circuit stub and is an in integral
part of the matching network. Having an excessively long trace to
the port of the reflective SPDT switch would reduce the usable
bandwidth of the antenna. In a triple or quad-band configuration,
all unused feed strips act in a similar way and have to be
carefully taken into account. In the embodiments presented here,
the beam selection switches are located inside the hallow CIFA
element with ventilation added to facilitate simultaneous reflow
soldering of the CIFA and the switches located inside. Lastly, it
should be noted that the RF switches can be located either inside
or outside of the RF shields as they see the substantially the same
signal as the antenna itself.
[0053] Diversity antenna control algorithms that can be used are
well known in the art. One example is provided by U.S. Pat. No.
8,305,270, the contents of which are herein incorporated by
reference. This uses constellation metrics and signal quality for
antenna selection.
[0054] Unlike the system described in U.S. Pat. No. 8,305,280 and
similar prior art, embodiments of the present invention use the
concept of system priority in its beam selection algorithm. Because
of the nature of GNSS systems, their satellites are well
distributed across the solid angle captured by the antenna. This
makes GNSS systems resistant to the loss of some fraction of the
captured solid angle. In contrast, because GEO systems typically
rely on a single satellite, they are much more susceptible to
degraded gain in a single line of sight. Embodiments of the present
invention map this resilience/susceptibility to priority level to
the antenna selection algorithm.
[0055] In the preferred embodiment, priority is given to the GEO
system, because it is a single satellite system that can benefit
from a tilted beam and because of its more constrained link
budget.
[0056] The antenna selection algorithm carried out in processor 19
is shown in FIG. 9. Upon receiving a starting stimulus at 20, for a
2-antenna system as shown in FIG. 2, the process starts at step 21
by measuring the received signal strength (RSSI) on antenna 2a
(ANT1). If the RSSI meets a predetermined criterion at step 22, in
this case considered ideal, the processor 18 commands the switch 13
to connect antenna 2a to the RF front-end module 14 for satellite
communications at step 24.
[0057] If at step 22 the RSSI does not meet the predetermined
criterion, the processor 18 commands the module 14 to measure the
RSSI on antenna 3 (ANT2) at step 24.
[0058] At step 25, the processor determines which RSSI is best and
connects the GEO module 14 to the corresponding antenna at steps
26, 27.
[0059] The process can be repeated at regular intervals or
alternatively triggered in response to signal degradation, for
example, due to the motion of a vehicle on which the antenna
assembly is mounted.
[0060] In this embodiment, the GNSS system shares the antenna that
was selected for the GEO system in a half-duplex fashion. The GEO
system shares the receiver front-end with the GNSS system, but when
the GEO system transmits, the receiver front-end is disconnected.
In this embodiment, transmissions generally scheduled not to
conflict with GPS and are short in duration to reduce possible
impact on GPS performance in cases where schedule accommodation is
not possible. An alternative approach to deal with longer
transmissions would be to have the GNSS system use the opposite
antenna from the GEO system, to avoid disconnecting the GNSS system
during transmit.
[0061] Another important consideration is frequency and bandwidth.
By providing two feed strips 8, 9 the antenna can be optimized over
two sub-bands. FIG. 10 shows the frequency response for the
different feed strips. The peak (minimum reflectance) shifts for
the different cases where the antenna is fed through the different
feed strips.
[0062] In a preferred embodiment, the higher priority GEO system
operates from 1518 MHz to 1675 MHz, which requires almost 10%
bandwidth. By making the antenna tunable, it can be stepped across
the frequency band to cover the frequency band, despite its limited
instantaneous bandwidth.
[0063] It will thus be seen that embodiments of the invention
provide a system that makes use of both GEO (such as Inmarsat)
satellites and non-GEO GNSS satellite constellations (such as GPS,
Galileo, GLONASS) and employs a multi-element, multi-beam antenna
array with elements that have substantially isotropic radiation
patterns but whose patterns are tilted to favor radiation in
directions opposite to each other.
[0064] A beam selection algorithm selects the optimal antenna based
on signal strength, wherein priority is given to the GEO system.
The systems results in the low elevation antenna gain of the array
over 360 degrees of azimuth exceeding the gain that would be
achieved by a single element, while maintaining sufficient
omnidirectionality to avoid degraded non-GEO system
performance.
* * * * *